Communication devices such as terminals are also known as e.g. User Equipments (UE), wireless devices, mobile terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network, also referred to as wireless communication system, cellular radio system or cellular network. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Terminals may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cells, wherein each cell being served by an access node such as a Base Station (BS), e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. Evolved Node B, base station (“eNB”, “eNodeB”, “NodeB”, “B node”), or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations, based on transmission power and thereby also cell size, may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the wireless device. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the wireless device to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
Cell Reselection
Cell reselection or simply reselection may be used by the UE for selecting a target cell, typically in low activity Radio Resource Control (RRC states). But it may also be used in some moderate to high RRC activity states. Examples of low activity RRC states are idle state in Evolved Universal Terrestrial Radio Access (E-UTRA), idle mode in Universal Terrestrial Radio Access (UTRA), Cell Paging CHannel (CELL_PCH) state in UTRA, Universal Terrestrial Radio Access Network (UTRAN) Registration Area Paging (CHannel URA_PCH) state in UTRA and Cell Forward Access CHannel (CELL_FACH) state in UTRA. The CELL_FACH state in UTRA may also be regarded as a moderate RRC state in terms of traffic activity. Nevertheless, in all these states, the cell reselection may be performed by the UE.
The target cell may typically be the strongest cell in terms the received signal quality at the UE, or it may also be a cell associated with higher priority, etc. . . .
Priority Based Cell Reselection
Absolute priority cell reselection, also referred to herein as absolute priority reselection, was defined in 3GPP TS 36.304, release 8, version 8.8.0, and TS 25.304, release 8, version 8.8.0, for both UTRA and Evolved Universal Terrestrial Radio Access (E-UTRA) inter-frequency reselection, as well as reselections between UTRA, E-UTRA, Global System for Mobile communication (GSM) and Code Division Multiple Access 2000 (CDMA2000), e.g., CDMA2000 1× Round-Trip Time (RTT) and High Rate Packet Data (HRPD). In the future, cell reselection between UTRA or E-UTRA and Wireless Local Area Network (WLAN) may also be introduced. In absolute priority reselection, each frequency layer may be assigned an absolute priority by the UTRA or E-UTRA network. ‘Layer’ is interchangeably used with other similar terms such as carrier frequency, frequency layer and carrier. But all of them bear the same meaning, namely, a group of cells which may be measured by a wireless device. For example, for GSM, a group of up to 32 cells may be considered a layer. Further description for layer will be provided later. An absolute priority may be lower, equal or higher compared to the priority of the serving frequency layer. The priority is so-called absolute because it is assigned in terms of absolute numbers, e.g., 0 to 7.
When the serving cell signal strength and quality are good, the UE may only search for layers with a higher priority than the serving layer, and this search is done relatively infrequently. The minimum requirement to perform this search may be every Thigher_priority_search*Nlayers according to TS25.133, v8.10.0, and TS36.133, v8.10.0. Thigher_priority_search is a search time for the higher priority search fixed at 60 s by TS25.133, v8.10.0 and TS36.133, versions 8.10.0, and Nlayers is the number of higher priority layers which have been configured.
The signal strength, e.g., Srxlev in TS 25.133, v8.10.0, or TS 36.133, v8.10.0, and signal quality, e.g., Squal in TS 25.133, v8.10.0, or TS 36.133, v8.10.0, may be derived from UE measurements and additional parameters signaled by the network node. Examples of UE measurements used for deriving signal strength and signal quality for cell reselection procedures are Common Pilot Channel (CPICH) Received Signal Code Power (RSCP) and CPICH chip Energy/Noise (Ec/No) respectively in UTRA, and Reference Signal Received Power (RSRP) and Reference Signal Received Quality (RSRQ) respectively in E-UTRA.
When the serving cell signal strength or quality go below configured thresholds, e.g., Srxlev≦SnonIntraSearchP or Squal≦SnonIntraSearch in TS 25.133 v8.10.0 or TS 36.133 v8.10.0, then the UE may search for and measure inter-frequency layers of higher, equal or lower priority in preparation for possible reselection. The thresholds SnonIntraSearchP and SnonIntraSearchQ may be typically configured, such that UEs may start to perform non intrafrequency measurements before the serving cell signal strength and quality fails to meet suitability criteria, as defined in TS25.304, v8.5.0, and TS36.304, v8.5.0, respectively. Cell detection and measurement on frequency layers, of higher, equal or lower priority, is performed at a greater intensity than the high priority search, since the UE may be assumed to be reaching the edge of coverage of the serving frequency. In this scenario, according to existing methods, all layers are currently considered equally as candidates for reselection. Therefore, for coverage reselections, when e.g., Srxlev≦SnonIntraSearchP or Squal≦SnonIntraSearchQ, the aforementioned cell detection and evaluation delays may be scaled by the total number of carriers on each Radio Access Technology (RAT), regardless of priority.
With the existing methods for cell reselection, UEs may go out of service before the cell reselection process is completed. Once a UE is out of service, it may not get any cellular service until it has performed cell selection procedures, which may correspond to a significant delay, and there may also be additional signaling load when the UE finds a suitable cell and performs registration.